HTTPTransaction: Move the transportCallback_
HTTPTransaction: Move the transportCallback_

Summary: Also add a couple of DestructorGuards.  We're seeing a crash
in android from transport callback.  It looks like there are some
semantic issues, but we aren't convinced this is the root cause.

Reviewed By: @francis-ma

Differential Revision: D2392638

pueril/proxygen,hiproz/proxygen,songfj/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,Orvid/proxygen,pueril/proxygen,hiproz/proxygen,pueril/proxygen,Orvid/proxygen,pueril/proxygen,songfj/proxygen,KublaikhanGeek/proxygen,hiproz/proxygen,Orvid/proxygen,KublaikhanGeek/proxygen,hiproz/proxygen,songfj/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,songfj/proxygen,KublaikhanGeek/proxygen,KublaikhanGeek/proxygen,Orvid/proxygen